Field of the Invention
The present invention relates to a method and apparatus for efficient utilization of a gas re-circulation selective catalytic reduction (SCR) system with heat trace. 2. Background of the Invention
Selective catalytic reduction (SCR) reactor technology is being used with increasing frequency to treat the exhaust gases from an industrial process, such as energy production, before the gas is released into the atmosphere. The SCR reactor process relies on the use of a catalyst to treat the exhaust gas as the gas passes through the SCR reactor. Because the catalyst is an integral part of the chemical reaction, great effort is used to provide maximum exposure of the catalyst to the exhaust gas and to ensure that all the exhaust gas comes sufficiently into contact with the catalyst for treatment.
The combustion of fossil fuels, such as coal, oil, and industrial or natural gas produces environmentally hazardous substances, including nitrogen oxide (NO) and nitrogen dioxide (NO.sub.2). Nitrogen oxide and nitrogen dioxide are collectively called NO.sub.x. In the normal combustion process of fossil fuels, the major portion of NO.sub.x is NO. The production of NO.sub.x can occur when fossil fuel is combusted in a variety of apparatuses, including refinery heaters, gas turbine systems, and boilers, such as in steam plants. The fuel may include coal, oil, gas, waste products, such as municipal solid waste, and a variety of other carbonaceous materials.
There are a number of known NO.sub.x reducing agents. A commonly used NO.sub.x reducing agent is ammonia. The principal process for the removal of NO.sub.x from the flue gas flow is the injection of a reducing agent, such as ammonia, urea, or any of a number of other known reducing agents, into the flue gas flow. For example, the selective catalytic reduction of NO.sub.x involving the injection of ammonia (NH.sub.3) into a flue gas flow in the presence of a catalyst occurs as the following chemical reactions: EQU 4NO+4NH.sub.3 +O.sub.2.fwdarw.(with catalyst)4N.sub.2 +6H.sub.2 O; and EQU 2NO.sub.2 +4NH.sub.3 +O.sub.2.fwdarw.(with catalyst)3N.sub.2 +6H.sub.2 O.
One method of injecting ammonia into a flue gas flow utilizes an external ammonia vaporization system in which liquid ammonia (either in anhydrous or aqueous state) is vaporized in a heater or vaporizer, mixed with air, and then routed to a distribution/injector grid for injection into the flue gas flow at a location "upstream" of an SCR reactor. Because anhydrous ammonia is toxic and hazardous, the general practice is to use a mixture of ammonia and water (NH.sub.3.H.sub.2 O). Ammonia diluted with water, i.e., aqueous ammonia, is less hazardous than anhydrous ammonia. A typical industrial-grade aqueous ammonia contains approximately 30% ammonia and 70% water. The ammonia-water mixture of the above mentioned percentages is safely transported on highways, and it has negligible vapor pressure at ordinary temperatures.
For the SCR systems that utilize ammonia, there are several ways to vaporize the ammonia. One method utilizes an electric heater to heat ambient air and mix it with aqueous ammonia in a vessel, thus vaporizing the aqueous ammonia. Another method utilizes a kettle-type heat exchanger tank in which a tank is filled with aqueous ammonia. The tank contains coils that are supplied with steam to vaporize the aqueous ammonia. Still another method utilizes an ammonia stripping tower in which aqueous ammonia is sprayed into the top of a fluid-fluid type contact tower, and steam is introduced into the bottom. A fourth method utilizes a flue gas slip-stream that is drawn by a blower into a vaporizer vessel where the flue gas mixes with and vaporizes the aqueous ammonia. FIG. 1 illustrates a prior art gas re-circulation selective catalytic reduction system. Hot flue gas is drawn into the vaporizer 100 by a dilution fan 130, and the reducing agent is vaporized to form a diluted reducing agent mixture, which is sent to the injector 160 for injection into the gas flow within the boiler 190.
U.S. Pat. No. 5,296,206, Using Flue Gas Energy to Vaporize Aqueous Reducing Agent for Reducing of NO.sub.x in Flue Gas, teaches yet another method where ambient air is drawn by a fan into an inlet. The air then travels through a heating tube, located in a boiler that is exposed to a hot flue gas flow within the boiler. The hot flue gas flow heats the heating tube, which in turn heats the air moving through the inside of the heating tube. The heated air eventually moves into a vaporizer. The heated air is mixed with the ammonia, and the mixture is sent to the injection grid for injection into the hot flue gas flow to perform the reduction reaction.
In order to prevent the formation of NH.sub.4 HSO.sub.4 (ammonium bisulfate), which occurs when sulfur-bearing fuels are burnt, in the pipes of the gas re-circulation selective catalytic reduction system, the heated air entering the vaporizer must be very hot. If the temperature of the air moving through the gas re-circulation selective catalytic reduction system is above 250 degrees Celsius, ammonium bisulfate does not form at meaningful rates. Ammonium bisulfate is a sticky and corrosive substance that damages the equipment upon which it is formed. Therefore, it is preferable that when the diluted ammonia mixture exits the vaporizer that the temperature of the mixture be greater than 250 degrees Celsius in order to prevent the formation of ammonium bilsulfate on the insides of the pipes carrying the mixture.
Electric air heaters may be used to raise the air temperature within the vaporizer and thus raise the temperature of the diluted ammonia mixture exiting the vaporizer in order to prevent the formation of ammonium bisulfate on the pipes "downstream" from the vaporizer. However, the power consumption of these electric heaters is very high, as are the maintenance costs.
A more common system for raising the air temperature of the diluted ammonia mixture is to utilize the hot flue gas, or hot combustion air, from the boiler to mix and vaporize the ammonia to produce the diluted ammonia mixture. The diluted ammonia mixture is then distributed into the hot flue gas flow in the boiler to perform the reduction reaction. The hot flue gas system is known as a hot gas re-circulation system and is the most common system used in conjunction with aqueous NH.sub.3. The hot gas re-circulation system extracts the hot flue gas from a boiler or gas turbine by way of a fan. The fan delivers the hot gas to the vaporizer. Two main factors determine the amount of hot gas required: (1) the vaporizer inlet temperature (extraction temperature); and (2) the vaporizer outlet temperature. Typically, the vaporizer inlet temperature is a fixed value of about 370 to 399 degrees Celsius (700 to 750 degrees Fahrenheit) (the typical temperature of the hot flue gas within the boiler); and the vaporizer outlet temperature is a temperature determined to prevent the formation of ammonium bisulfate when sulfur-bearing fuels are burnt in the boiler or gas turbine, typically at a temperature of at least 250 degrees Celsius (482 degrees Fahrenheit).
Hot gas re-circulation systems require large fans to propel the hot flue gas into the vaporizer, as well as large pipes in order to carry the hot gases and mixtures. Larger fans have higher power consumption and maintenance costs than smaller fans, and larger pipes have greater costs than smaller pipes.